The invention relates to a discharge lamp for generating electromagnetic radiation equipped with a gastight discharge vessel that contains a filling that comprises a gas. The vessel is transmissive for at least part of the electromagnetic radiation that is generated by the filling during operation, and is equipped with lamp electrodes between which a discharge is maintained during operation
The invention also relates to a lighting arrangement comprising a discharge lamp and a ballast circuit.
An example of such a discharge lamp is a low pressure mercury discharge lamp, also called fluorescent lamp, as for instance described in "Electric discharge lamps" by J. F. Waymouth, M.I.T. Press, Cambridge 1971. In such a fluorescent lamp the filling comprises apart from a noble gas also mercury and the wall of the gastight discharge vessel is covered with a luminescent layer that during operation converts the UV-radiation generated in the lamp into visible radiation. The lamp electrodes are normally present in the discharge vessel and consist of a metal alloy covered with an emitter material. It has been found that such fluorescent lamps can be operated with high efficiency (1 m/W) by means of a high frequency current. Such a high frequency current is often generated out of a supply voltage supplied by a supply voltage source by means of ballast circuit comprising a DC-AC-converter. Because of the relatively high voltages that are needed to ignite the fluorescent lamp and maintain the discharge during stationary operation, the DC-AC-converter is often equipped with a transformer. In such a DC-AC-converter the lamp is coupled to the secondary side of the transformer. Such a transformer can be an inductive transformer or a piezotransformer. DC-AC-converters that are equipped with a piezotransformer offer substantial advantages. First of all, piezotransformers can be very flat and small so that the ballast circuit can also be flat and small, which is very important in case the ballast circuit is for instance used to operate a lamp that serves as a backlight in a flat panel display. Furthermore, since piezotransformers have a positive current-voltage relation a separate ballast choke can often be dispensed with. Another important advantage is that the properties of the piezotransformer are strongly dependent on load that is coupled to its secondary side. In practice it has been found that because of this strong dependency it is possible to generate an ignition voltage having a relatively high amplitude (e.g. 1-1.6 kV) and a much lower lamp voltage (e.g. approximately 500-800 Volt) that maintains the discharge during stationary operation by applying the same signal to the primary side of the piezotransformer. This means that the DC-AC-converter need not comprise additional circuitry for changing the signal present at the primary side of the piezotransformer upon ignition of the lamp and can therefore be relatively simple and cheap. Despite these advantages, a lighting arrangement that comprises such a known discharge lamp and a ballast circuit for operating the discharge lamp comprises many components.